Diabatic processes and the structure of the warm conveyor belt

Oscar Martínez-Alvarado

J. Chagnon, S. Gray, R. Plant, J. Methven

Department of Meteorology

University of Reading

H. Joos, M. Bötcher, H. Wernli

ETH Zurich

2nd European Windstorm Workshop

Leeds, 3-4 September 2012

• Diabatic processes are fundamental to clouds and

precipitation (i.e., weather)

• In NWP models these processes are parameterized

• The nonlinear feedback between the cloud scale and

larger-scale dynamics has implications for:

Forecasts of heavy precipitation and high wind events

Assimilation of high resolution data (e.g., radar)

Linking forecast error to model representation of processes

Diabatic (heating) effects on medium-range forecasts

Design of perturbed physics ensembles

Why study diabatic processes in ET storms?

Systematic error (PV overestimated) in medium-range

weather forecasts on the downstream side of troughs.

Diabatic PV near the tropopause

Objectives

• Evaluate the accuracy of numerical models in

simulating atmospheric diabatic processes

– Extratropical cyclones and more specifically the warm

conveyor belt (WCB) constitute the focus of this study

• What diabatic processes act on the WCB?

• What effect do these processes have on the WCB

development?

• What are the consequences for the subsequent

development of the upper-level atmospheric structure?

Warm conveyor

belt

Satellite image courtesy of NERC Satellite Receiving Station,

Dundee University, Scotland http://www.sat.dundee.ac.uk

Surface fronts based on the Met Office analysis at 00 UTC 25

Nov 2009 (archived by http://www.wetter3.de/fax)

Cyclone’s low

pressure centre

Cold conveyor

belt

Channel 22, satellite

Aqua

0247 UTC 25 Nov 2009

Methods

0

proc

( , )( , ) ( , )i

i

x t xt tx

Tracers (I)

Tracers (II)

·

i

i ii

DS

Dt t

v

0 00

· 0

D

Dt t

v

The conserved field is

affected by advection

only

= proc = p

0

roc

· ·

ii

i i

St

v v

Total rate of change

Advection of

conserved field

Advection of accumulated

tendencies

Sources

Consistency between tracers

and trajectories

Numerical models and

simulations

• Two numerical weather forecast models have been

used

• Reading: The Met Office Unified Model (MetUM)

– 12 km resolution

• ETH-Zürich: The COSMO (COnsortium for Small-scale

Modelling) model

– 14 km resolution

• Both simulations initialised at 0600 UTC 23 November

2009 from ECMWF operational analysis fields.

Case-study:

Synoptic-scale context

• The surface low formed in the North Atlantic on 23 November 2009 along an

east-west oriented baroclinic zone

• The low deepened from 0000 UTC 23 November to 0000 UTC 25 November

2009 and moved eastward.

• By 0000 UTC 25 November, the system was occluded and had undergone

“frontal fracture”.

• Precipitation was heavy and continuous along the length of the cold front

during the period 23-25 November 2009. As such, this is an ideal case for

examining diabatic heating in a WCB.

• The upper-level trough associated with the primary low amplified in concert

with the surface low.

• The downstream ridge and downstream trough also amplified during this

period.

Trajectory selection

Wide starting

region to capture

every ascending

trajectory related

to the system

A

B

C

Trajectory bundle

Identification of sub-streams

θ < 307.5 K θ > 307.5 K

X grid point X grid point X grid point

Y g

rid

poin

t

All trajectories

Lower branch Upper branch

B + C A

Potential temperature conserved

component (K)

Model level at 9.68 km

Upper-level structure (I)

Potential temperature (K)

Diabatic potential vorticity (PVU) Diabatic potential temperature

(K)

1

0.5

0

-0.5

2

Model level at 9.68 km

Upper-level structure (II)

A B + C

D

Dt

(K h

-1)

Pressure

(hPa)

Pressure

(hPa)

lscD

Dt

(K h

-1)

Heating rates – MetUM (I)

Total

heating

Total

heating

Large-

scale cloud Large-

scale cloud

A B + C

D

Dt

(K h

-1)

(K h

-1)

Heating rates – MetUM (II)

Pressure

(hPa)

Pressure

(hPa)

bllhD

Dt

Total

heating

Total

heating

Boundary

layer Boundary

layer

A B + C

D

Dt

(K h

-1)

(K h

-1)

Heating rates – MetUM (III)

Pressure

(hPa)

Pressure

(hPa)

convD

Dt

Total

heating

Total

heating

Convection Convection

A + B C

D

Dt

Heating rates – COSMO model

Pressure

(hPa)

Pressure

(hPa)

Summary and conclusions

• The WCB splits in two branches in its outflow region

• The upper-level structure reflects and is affected by

this split

• Diabatic processes act differently over these two

branches

• The upper-level structure is modified by these diabatic

processes (through the WCB split)

Future work

• Complete a systematic comparison between the two

models

– Further integrate the diabatic decomposition techniques at both

research centres

• The formation of a streamer at the tip of the downstream

ridge over North Africa may be noteworthy? Did this lead to

heavy precipitation downstream in the eastern Med?

– A MetUM forecast of the upper-level trough and ridge structure

only departed from the analysis in one noteworthy respect -- the

downstream ridge extended too far northwards .

– A second MetUM forecast was also performed but not discussed

here. This forecast, which was initialised 30 hours earlier, was

much less accurate than the one presented here (e.g., the

downstream ridge was not elongated and did not lead to

streamer formation)

References

• Grams, C. M., Wernli, H., Bötcher, M., Čampa, J.,

Corsmeier, U., Jones, S. C., Keller, J. H., Lenz, C.-J.

and Wiegand, L. (2011) The key role of diabatic

processes in modifying the upper-tropospheric wave

guide: a North Atlantic case-study. Q. J. R. Meteorol.

Soc. 137: 2174–2193.

• Joos, H. and Wernli, H. (2011) Influence of

microphysical processes on the potential vorticity

development in a warm conveyor belt: a case-study

with the limited-area model COSMO. Q. J. R.

Meteorol. Soc. 138: 407–418